The present invention relates to a device for measuring the relative rotational speed between the races of a rolling contact bearing.
There are known rolling contact bearings fitted with a sealing device comprised of a metal annular insert mounted to the rotating race of the bearing. In some applications, such insert forms a radial flange on the outer wall of which there is fixed, in different manners, a multipolar ring (encoder wheel) with magnetized segments of alternate polarity in the circumferential direction. The encoder wheel forms part of a device for measuring the relative rotational speed between the bearing races. The encoder wheel faces a sensor secured to a stationary part of the bearing and responsive to variations of the magnetic field generated by the rotating encoder wheel.
EP-A-0 607 719 discloses a sealing device for bearings incorporating a magnetized ring. The axially outer surface of a metal insert of the sealing device is covered with an elastomeric material charged with metal magnetic particles which also has a sealing function. The cover layer of elastomeric material must be of a relevant thickness in order to generate a sufficiently strong magnetic field to be sensed by the sensor device.
A variant solution provides for a rubber or elastomeric lip being vulcanized onto the annular metal insert. A further layer of elastomeric material charged with magnetized particles is then vulcanized covering the axially outer radial surface of the first rubber or elastomeric material. This solution, while having the limits of the previously discussed one, further requires a complicated and expensive manufacturing process.
EP-A-0 725 242 discloses an elastomeric encoder wheel glued to a support metal insert.
In other known solutions, for example as mentioned in EP-A-0 647 851, a plastic ring containing metal particles (plastoferrite ring) is co-moulded or glued onto the metal insert. The use of plastoferrite is advantageous in that it allows to obtain higher magnetization levels as compared to conventional elastomeric rings charged with magnetized particles. However, the use of plastoferrite has a drawback concerning the difference of the thermal dilatation coefficients of the plastic forming the encoder ring and the steel of the metal insert. Therefore, this solution is not applicable with motor vehicle wheel hub bearing units, where the heat generated by the near brakes causes thermal variations compromising the adherence between the encoder wheel of plastoferrite and its steel supporting annular disc.
Against the foregoing background, it is the primary object of the present invention to provide an improved encoder wheel capable of reaching higher magnetization levels while overcoming the above prior art drawbacks due to thermal variations.
Toward the attainment of these and additional objects and advantages, the present invention, briefly summarized, provides a device for measuring relative revolving speed between the races of a rolling contact bearing fitted at one end thereof with a sealing device, wherein said sealing device comprises a metal insert of annular shape fast for rotation with the rotating race of the bearing, and the speed measuring device comprises a multipolar ring mounted to said metal insert and provided with magnetized segments of alternate polarity in the circumferential direction, characterized in that said multipolar ring is a ring of plastoferrite co-moulded to said insert, and that on said insert there is formed a plurality of radial recesses adapted to yield, in the co-moulding operation, a corresponding plurality of radial protrusions in the multipolar ring, said radial recesses and protrusions being mutually engaged so as to prevent relative rotation between the multipolar ring and the metal insert, but allow relative radial displacement owing to thermal variations.